1. Field of the Invention
The present invention generally relates to charge transfer devices and, more particularly, is directed to a charge transfer device suitable for use in a charge-coupled device (CCD) delay line having a comb filter characteristic.
2. Description of the Prior Art
A video signal processing system uses a comb filter to to separate a chrominance signal (carrier chrominance signal) from a composite video signal.
From a principle standpoint, the comb filter delays a composite video signal of the NTSC system by one horizontal scanning period (1H) and then subtracts the delayed composite video signal and a succeeding 1H-delayed composite video signal from each other to thereby separate the chrominance signal from the composite video signal. In the case of the PAL video signal, a chrominance signal is separated from a composite video signal by delaying the composite video signal by two horizontal scanning periods.
The comb filter having the aforementioned function can be realized by using a CCD delay line. The conventional comb filter with the aforementioned function can change its characteristic, for example, selectively change its characteristic between a comb filter characteristic with an 1H-delay amount and a comb filter characteristic with a 2H-delay amount in a manner that two CCD delay line respectively used as comb filters with the 1H- and 2H-delay amounts are contained in an integrated circuit (IC) and one of outputs of the two CCD delay line is selectively output.
An example of this type of the conventional comb filter will be described with reference to FIG. 1. Referring to FIG. 1 of the accompanying drawings, video signals INA and INB for obtaining the comb filter characteristic are input to each of two shift register sections 31 and 32 each constituted by a CCD and respectively serving as the comb filters with 1H- and 2H-delay amounts.
In the CCD register sections 31 and 32, for example, in the CCD register section 31, the video signal INA is converted into electric charge (i.e., so-called signal charge) and then delayed by an amount D and the video signal INB is converted into signal charge and then delayed by an amount (1H+D), and both the signal charge with the delay amount D and the signal charge with the delay amount (1H+D) are added, thereby the comb filter characteristic with an 1H-delay amount is obtained.
In general, in order to separate the chrominance signal from the composite video signal in the form of the NTSC system, the video signals with opposite phases shown in FIGS. 2A and 2B are input to the CCD register section 31 as the video signals INA and INB, respectively.
Accordingly, an adding operation is performed in the form of signal charge between the image signal INA and the image signal INB which has a phase opposite to that of the image signal INA and is delayed by 1H with respect to the image signal INA. As a consequence, only the chrominance signal is obtained from the output of the CCD register section 31 as shown in FIG. 2C, so that the separation of the chrominance signal can be realized. One of the outputs of the CCD register sections 31 and 32 is selectively applied to an output circuit 33 through a selection switch SW1 and then output from the output circuit 33 as an output signal. In this case, the changing operation of the switch SW1 is performed in accordance with a control signal applied thereto.
According to the aforementioned prior art technique, however, the changing operation between the comb filter characteristic with an 1H-delay amount and the comb filter characteristic with a 2H-delay amount is performed at the output sides of the CCD register sections. In this regard, since different signals are developed at the outputs of the CCD register sections 31 and 32 simultaneously, a crosstalk occurs between both the outputs, so that an accurate comb filter characteristic may not be obtained due to the crosstalk.
The crosstalk may be eliminated by modifying a layout of a circuit pattern from the CCD register sections to the switch SW1 or modifying the construction of the switch SW1 or the like, which, however, results in the increase of a size of the IC chip.